Roberto Aloi has put together a tool for Emacs that allows you to directly browse the PHP documentation without hopping out of the editor.

php-mode is a set of tools to increase productivity in Emacs when programming PHP. It can be used in conjunction with any other pre-existing PHP mode for Emacs.

You can see an example of it in action in this screenshot and can install it via the five or six steps included in the page (that include downloading the latest "many HTML files" option from the PHP.net documentation page).